Make sure that the current working copy and the working copy
dir (if PATH) provided by the head parameter belong to the same
repository.

* contrib/client-side/svnmerge.py
  (wc_of_same_repos): New function. Given two working copy dirs,
  read the entries file, and make sure that both of them belong
  to the same repository.
  (main): Add check to make sure that the provided head parameter
  and the branch dir are working copies and belong to the same
  repository.